Method and apparatus for positioning a device in a body

ABSTRACT

There is disclosed a method of positioning an interventional device in a body using a guide pivoting about a pivot point, performed by locating the spatial coordinates of a target and the pivot point, determining a third point outside of the body lying along or proximate a line extending through the target and pivot point, and aligning the axis of the guide with the third point using an imaging system. There is also disclosed a medical imaging system including a processing unit and computer software operative on the processing unit to permit an operator of the system to locate the spatial coordinates of a target point and a pivot point of a guide, and determine a third point outside of the body lying along or proximate a line extending through the target and pivot point. This medical imaging system may further include computer software operative on the processing unit to assist an operator in obtaining an image by which the axis of the guide can be aligned with the third point using an imaging system. There is also disclosed an article of manufacture formed by a computer program encoded in a carrier, wherein the program is operative on a processing unit of a medical imaging system to permit an operator of the system to locate the spatial coordinates of a target point and a pivot point of a guide, and determine a third point outside of the body lying along or proximate a line extending through the target and pivot point.

This application is a continuation-in-part of U.S. application Ser. No.09/168,792 filed Oct. 8, 1998. U.S. Ser. No. 09/168,792 is incorporatedby reference herein.

TECHNICAL FIELD OF THE INVENTION

The present invention pertains generally to the field of medicine, andmore particularly to positioning an interventional device in a bodyusing a medical imaging system.

BACKGROUND OF THE INVENTION

Computed tomography (CT)-guided biopsies have been performed since theearly days of CT scanning when it became apparent that the crosssectional imaging modality offered unprecedented abilities to visualizethe needle in cross section to verify positioning within a lesion. Overthe last 15 years, the methodology for the CT-guided biopsy has remainedlargely one of trial and error. Essentially, a scan of the appropriatebody part is made and a mental calculation of the trajectory is madefollowing a depth calculation on the computer console. The depth is thentransferred to the interventional device which has been marked. Theinterventional device is then inserted, removed, and reinsertedrepeatedly with repeat scanning at the appropriate interventional deviceposition to confirm proper placement or improper placement. Obviously,this technique of trial and error introduces undesirable delays, risks,costs and, in some cases, exposure to unwanted radiation.

In addition to the matter of CT-guided biopsies, there has been muchrecent work in the field of MR-guided surgery, including biopsies andother minimally-invasive procedures. At present, methods of trajectorylocalization under MR are based largely on frameless stereotacticconcepts. While this is a feasible methodology for many situations,there remains an issue of cost. To date, there has not been a methodproposed that is simple, accurate, and inexpensive for use in the MRsetting.

Therefore, there remains a need for a method for locating ainterventional device in a body part which is faster and moreconvenient.

SUMMARY OF THE INVENTION

According to one example embodiment, the present invention provides amethod of positioning an interventional device in a body using a guidepivoting about a pivot point, comprising locating the spatialcoordinates (or the image display of a point corresponding to saidcoordinates, even if said coordinates are not explicitly stated becausea computer is capable of interpreting the mathematical relationship ofthe display to the true coordinates) of a target and the pivot point,determining a third point outside of the body lying along or proximate aline extending through the target and pivot point, and aligning the axisof the guide with the third point using an imaging system.

According to another example embodiment, the invention provides amedical imaging system including a processing unit and computer softwareoperative on the processing unit to permit an operator of the system tolocate the spatial coordinates of a target point and a pivot point of aguide, and determine a third point outside of the body lying along orproximate a line extending through the target and pivot point. Thismedical imaging system may further include computer software operativeon the processing unit to assist an operator in obtaining an image bywhich the axis of the guide can be aligned with the third point using animaging system.

According to another embodiment, the invention provides a method ofusing the MR signal from one or more radiofrequency microcoils placed onthe trajectory alignment stem at the pivot point and at the at leastthird point to determine the spatial locations of these two coils, andhence the position of the alignment stem, including its orientation.Moreover, with this information determined and therefore known to the MRscanner computer, the trajectory alignment stem could be realigned tomatch the desired trajectory, either manually, by remote or roboticcontrol, or by control of the MR scanner computer itself, by means of aninterface with a servo mechanism either directly or indirectly attachedor related to the trajectory alignment stem.

According to another embodiment, the invention provides an article ofmanufacture comprising a computer program encoded in a carrier, whereinthe program is operative on a processing unit of a medical imagingsystem to permit an operator of the system to locate the spatialcoordinates of a target point and a pivot point of a guide, anddetermine a third point outside of the body lying along or proximate aline extending through the target and pivot point.

According to yet another embodiment, the invention may provide that theaxis of the guide is aligned automatically under software control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C are MRI images illustrating one example method ofthe invention, with the image of FIG. 1B taken along the lines 1B—1B ofFIG. 1A.

FIGS. 2A and 2B illustrate post-alignment imaging views.

FIG. 3 is a flow diagram of an example method of the present invention.

FIGS. 4 illustrates a medical imaging system and software componentsaccording to an example embodiment of the invention.

FIG. 5 illustrates an example trajectory guide.

FIGS. 6 and 7 illustrate alternate embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings that form a part hereof,and in which are shown by way of illustration specific embodiments inwhich the invention may be practiced. It is understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

The present invention, as described below, provides method and apparatusfor aligning an straight (or substantially straight), elongate, pointpivoted interventional device to an orientation in a human or animalbody. As used herein, the term “interventional device” refers to anymedical instrument, such as a biopsy needle, probe or other type ofneedle (etc . . . ). The invention is described below in an exampleembodiment wherein it is applied to position an interventional device ina human brain. It shall be understood, however, that the invention is inno way limited to use in positioning interventional devices in thebrain, but can applied broadly to the positioning of interventionaldevices in any part of human or animal bodies.

Example Method Using Magnetic Resonance Imaging

The example embodiment set forth below provides a method of MRI-guidedbiopsy involving an intracranial lesion along a trajectory orientedsomewhat parallel to the long axis of the patient, as might be used fora biopsy of a high frontal lobe lesion. The long axis of the patient isthe axis that is generally coaxial to the length of the patient's body.This method is described with reference to FIGS. 1A, 1B and 1C, whichillustrate MRI images 2A and 2B, respectively. Either prior to, or, inone example embodiment, following, administration of an intravenouscontrast agent to the patient, a limited MRI scan is obtained throughthe head 5, particularly involving the region of the planned target 12.The benefit of first administering intravenous contrast is two fold.First, there is quite often contrast enhancement of the targeted lesionitself, making it easier to identify the lesion, and second, there isoften contrast enhancement of the cortical veins overlying the region ofthe planned trajectory. Since these are structures that are to beavoided if possible so as to minimize potential bleeding, it is helpfulto identify these veins prior to the determination of the trajectory.

Once the target 12 is identified, the trajectory is determined using thescanner. A point on the scalp is chosen for an entry point, therebycreating a surgical approach. The approach can be verified either byperforming a multiplanar reconstruction of the already obtained imagingdata, or by simply performing a new single slice image along the desiredtrajectory. If the latter method is used, the scan plane can be easilyadjusted, if necessary, until it is determined that the surgicalapproach is along the axis of the current scan plan. Alternatively, theentry point is determined by other means.

Once the entry point is determined, a trajectory guide 10 is surgicallyimplanted either on the surface of the calvarium or in a burr hole thatis drilled at this point. The design of trajectory guide 10 is notimportant to the invention, other than it includes, as illustrated inthe example guide 10 of FIG. 5, a guide member 17 that pivots about apivot point. As illustrated in FIG. 5, guide member 17 includes a lowerportion 17A which is pivotally connected to a base 16, and an upperportion 17B that can receive an interventional device, or a guide stem18 that is used during the alignment process. Further information on theexample trajectory guide 10 illustrated in FIG. 5, and information onother possible guides that may be used with the present invention, canbe found in U.S. patent application Ser. No. 09/078913, entitled “RemoteActuation of Trajectory Guide”, filed May 14, 1998. It is noted thatwhile the pivot point of example guide 10 is located proximate thesurface of the body, the pivot point may be above the surface of thebody, for example where it is suspended above or outside the body withan articulated arm.

Once the trajectory guide 10 and alignment stem 18 are in place and thetrajectory is in line with scanning plane, (which in FIG. 1A provides animage slice taken along a plane encompassing the target 12 and at leastthe base 16 of the of the trajectory guide 10), several spatial locationpoints in the 3D space of the MRI scan are located: one in the head, onenear or above the surface of the head, and one outside. First, the x, y,z coordinates of the target 12 are determined by the operator from theimages obtained in the initial MRT scan of the head. Second, the x, y, zcoordinates of the pivot point of the guide member 17 are alsodetermined by the operator from the MRI scan image(s).

Once these two points are known, there is determined mathematically aline 30 which extends from the target 12, through the pivot point of theguide member 17 and out into the space 32 outside the head 5. This line,which can be displayed on the scan plane (which is aligned with thesurgical approach) of the current or reconstructed image, represents thetrajectory required to reach the desired target with an interventionaldevice. According to one example embodiment of the invention, the line30 is drawn to extend away from the base 16 about two thirds of thedistance from the base 16 up to the free end of the stem 18. A point 34is then chosen along line 30. Point 34 should fall along a 3-Dcircumferential arc described by the alignment stem 18 of the trajectoryguide which, according to one example embodiment, contains a MRI-visiblemarker 19, positioned at the same approximate distance from the base 16as the distance of point 34 from the base 16, to make it clearly visiblein a new image along the plane of the current or reconstructed image.Stem 18 may be made entirely of a MRI-visible marker material, or themarker 19 may be limited to a segment of the stem 18. Point 34 may bechosen by the operator on the line 30 when drawn on the scanner display,or it may be determined mathematically without operator intervention.

It is worth noting that while the description herein is based onmathematical precepts, in reality, most modem CT and MR scanners, andother imaging equipment, no longer need the operator to manually type incoordinates, as described above. Most scanner consoles work like homecomputers with a mouse, in that a cursor can be dropped on the screen todenote a point, a line can be drawn on the operator console from onepoint to another, even without the operator knowing the true coordinatesof the points used to determine the line. Nevertheless, behind thescreen facade, the computer is in fact translating the very pointsdisplayed into spatial coordinates and following mathematical models.Thus, in practice, most operators of the methods presented herein willnot in fact be required to perform the proposed steps, but merely willneed to point and click with a computer mouse or similar device tocreate the geometric plan of trajectory described herein.

In addition to the simple method described above and as describedfurther below, it is possible to envision that the alignment of thetrajectory alignment stem could be carried out by other visualizationmethods, such as with laser, infrared, or light of other frequencies orother energy sources focused at a point in space determined by theoperator to be the location of point 34 described above. Moreover, it iseasy to envision that the method of recognition by the operator ofcloser alignment of the trajectory alignment stem to line 30 need noteven be visual, but could be audible, such that, for example, the repeatrate of a repeating beep might vary depending on the success or failureof various movements to align the stem (i.e. a slower rate of beeping,if moving the stem further away from line 30, a faster rate of beeping,if approximating line 30, and a continuous sound when the alignment isdeemed successful.

Using the x, y, z coordinates of point 34, as either calculatedautomatically or obtained from the point selected by the operator online 30, a scanning plane 36 is chosen to include point 34 even thoughthe target itself is not visualized within the plane. The point 34 isthen marked (38) on the trajectory guide alignment plane. Twodimensional (2D) sequential scans are then obtained along plane 36 (FIG.1B) with interactive positioning of the trajectory guide alignment stem18 until such time as the MRI-visible marker 19 on the alignment stem 18comes into view and is positioned at the desired x, y, z coordinates. Asillustrated in FIG. 1B, the stem 18 is proximate the mark 38, such thatstem 18 can be easily moved into place in alignment with mark 38 withreference to a single point in space, which, from the perspective ofFIG. 1B, requires alignment only with respect to “x” and “y” coordinateslying in the trajectory guide alignment plane. Once the stem 18 alignswith points “x” and “y”, it is known to be coaxially aligned with line30. The alignment stem 18 of the trajectory guide 10 is thus properlyaligned (see FIG. 1C), and the guide member 17 can be locked in place toallow for the insertion of a interventional device or other instrument.

Moreover, although it seems simplest to envision that the line bisectthe trajectory guide alignment plane, in reality, what is needed is onlythat the point of intersection be predictable, such that the true pointof intersection be determinable on the image of the trajectory guidealignment plane and the orientation of the trajectory guide alignmentstem seen in cross-section to its longitudinal axis may be adjusted soas to the bring the two points in alignment.

At this point, for purposes of verification, although not mandatory, oneor more repeat scans are obtained through the intended target andthrough the length of the trajectory guide. Preferentially, in the caseof MR imaging, scans of orthogonal (or approximately orthogonal) planeshould be performed to ensure proper alignment. Alternatively,orthogonal (or approximately orthogonal) multiplanar reconstruction canthen be performed on the operator console once again, which shouldclearly demonstrate the alignment stem 18 properly aligned along theintended trajectory to the target. Examples of such scans are shown inFIGS. 2A and 2B.

Once the trajectory guide is locked into position, the alignment stem 18is removed from the trajectory guide and either the twist drill or, if ahole has already been made through the calvarium, the interventionaldevice itself or other probe or instrument, depending on thecircumstances of the procedure, can be passed to a predetermined depthby measuring off the confirmatory scans or the multiplanarreconstruction. Likewise, repeat scans can be obtained through thetarget itself which either will include the trajectory, if the patient'shead was tilted to the extent that the trajectory guide is within theplane, or only to show the target with the arrival of the tip at thetarget on one of the sequential scans.

The methodology described in the example above pertained to a lesionthat was approached along the long axis of the patient. In fact, thesame methodology works equally well in other orientations to the longaxis of the scanner. In other words, a temporal lobe biopsy is likely tobe approached along an orientation perpendicular to the long axis of thepatient. In the former case, what is referred to below as “thetrajectory guide alignment plane” is oriented roughly perpendicular tothe long axis of the patient. In the latter case, this plane will beroughly aligned with the long axis of the patient, also commonlyreferred to as axial or transverse, or oblique axial or obliquetransverse, typically sagittal or coronal with respect to the patient,or somewhere between these two orientations.

Moreover, although it seems simplest to envision that the line bisectthe trajectory guide alignment plane, in reality, what is needed is onlythat the point of intersection be predictable, such that the true pointof intersection be determinable on the image of the trajectory guidealignment plane and the orientation of the trajectory guide alignmentstem seen in cross-section to its longitudinal axis may be adjusted soas to the bring the two points in alignment.

Example Method Using Computed Tomography

While the methodology described in the example above pertains tosurgical procedures performed under MR imaging guidance, the methodologycan be applied similarly to CT scanning guidance. In such a situation,it is preferable, although not mandatory, to utilize a spiral CTscanner, for the sake of time and efficiency. In the example of a brainlesion to be approached somewhat along the long axis of the patient, abaseline spiral CT scan is obtained, typically, although notnecessarily, following the injection of intravenous iodinated contrastmedia. The target is chosen from the axial images displayed on thescanner console and a surface entry point is selected on thescalp/skull. Once this is accomplished, a multiplanar reconstruction ofthe spiral (or non spiral) data set is performed and the methodologydescribed above for MR is followed. In this scenario, what is referredto below as “the trajectory guide localizing plane” is oriented roughlyperpendicular to the long axis of the patient, and actually is likely tobe scanning not through the patient at all, but through the air, beyondthe patient's head, but still through the trajectory guide stem 18. Oncealigned, a repeat spiral (or non-spiral) data set, although notmandatory, can be obtained through the patient and trajectory guide stem18, and (approximately) orthogonal multiplanar reconstructed imagesalong the length of the actual trajectory can be viewed, for purposes ofconfirmation.

In addition, the methodology can work equally well on CT for lesionsaccessed along trajectories oriented other than longitudinal to thepatient. However, CT is a different methodology than MRI and the methodproposed herein is not optimal, when used in the true axial plane(perpendicular to the long axis of the patient). Nevertheless, a minormodification of the typical scanning methodology does permit thistrajectory method to succeed even when used to access a lesion in theaxial plane. To accomplish this, three different methods are describedbelow.

First, the trajectory alignment stem 18 can be easily maneuvered withinthe plane of imaging. Certainly the target and device can both bevisualized in this plane, and as long as the full length of the needleis visualized, one can be assured that a relatively accurate approach islikely.

Second, the target 12 can be chosen by scanning in the axial plane,typically as a spiral acquisition over a volume of tissue, such thatmultiplanar reformatting can be performed. An entry point can then bechosen at some obliquity away from the original true axial plane, atwhich time the scan plane can then be angled, which is common to all CTscanners, such that the image plane will be at some reasonable angleaway from the planned trajectory (for example by way of illustrationonly, about 5 to 15 degrees from the true axial plane). In addition, thescanner can also be angled in the opposite direction to the angled scanplane, to obtain a trajectory guide alignment plane. Furthermore, oncethe trajectory guide is locked in place the patient may be moved forwardor backward on the gantry so that the trajectory guide alignment planeis positioned to view the target. Thus, the arrival of theinterventional device at the target may be determined using the sameimage plane as used to position the guide. In this scenario, however,the mathematical calculations described below still prevail and apredictable intersection of the trajectory and the trajectory guidealignment plane will be evident.

Finally, a third methodology provides that once the target and entrypoint are selected (in the axial plane), the gantry can be angled(again, for example by way of illustration only, about 5 to 15 degrees),such that the angle of the new scan plane is such that the trajectoryline and the trajectory alignment plane will intersect.

In the first and third methods, the trajectory can be truly axial to thepatient. In the second method, the trajectory itself is modified to anoblique axial approach. This is actually a typical scenario for a liverbiopsy where the ribs are often in the line of approach for an axialtrajectory.

Thus, as described above, the invention, in one example embodiment,provides the following method (wherein the steps are not necessarilyperformed in the following order), illustrated in FIG. 3:

1. The area of interest in the body is imaged to locate the target andapproach (40).

2. A trajectory guide is placed in position on, in or near the body. Thetrajectory guide pivoting around a fixed point which may or may not beproximate the surface of the body, but would typically be at the surfaceor outside the body (41).

3. The coordinates of the target and pivot points as well as theorientation of the imaging plane are determined (42).

4. The trajectory guide alignment stem is imaged in a plane having anorientation approximately normal to the direction of the guide, or suchthat the plane at least intersects with alignment stem when it isaligned with the desired line of approach. This is referred to below asthe “trajectory guide alignment plane” (43).

5. The intersection point of a line defined by the two points with thetrajectory guide alignment plane is determined (44).

6. The intersection point is displayed as an in-plane target point onthe trajectory guide image (45).

7. The image of the guide stem, viewed substantially from a view looking“down” its axis, is aligned to the in-plane target point on thetrajectory guide image (essentially by moving the stem in “x” and “y”directions until aligned), preferably but not necessarily in “real time”(46).

It is noted that the trajectory guide alignment plane need not bestrictly orthogonal to the line intersecting the target and pivot point.Rather, it is only required that the line not lie entirely in thetrajectory guide alignment plane, so that the stem of the guideintersects with the plane.

Example Embodiment of Imaging Software/Device

According to one embodiment of the invention, there is provided imagingsoftware operative on an imaging device to enable the above-describedmethod. As shown as a block diagram in FIG. 4, an imaging system 50includes imaging apparatus 52, computer processing unit 54 includingsoftware 56, and a display 58. Imaging apparatus 52 may be an x-rayimaging, a magnetic resonance imaging (MRI) imaging unit, or anultrasound imaging unit. Apparatus 52 supplies imaging data to computerprocessing unit 54, which processes the imaging data under control ofsoftware 56 to produce images that are displayed on display 58, oroutput to a image printing system (not shown).

According to this embodiment of the invention, software 56 includes oneor more software components 60 which provide a user interface andsupporting computer processing instructions to guide an operator throughthe application of the methods of the present invention. This userinterface and supporting processing instructions preferably include, ata minimum, operator tools to identify the coordinates of the target andpivot point of the guide, and to determine the line passing throughthese points and the intersection of that line with the trajectory guidealignment plane to ascertain and mark on the display the alignment pointon the imaging plane. These tools in one example embodiment include theability to paint or “drop” a cursor mark at the target and pivot pointusing a pointing device such as a mouse, to establish the coordinatesfor calculating line 30, which may be done automatically once the cursormarks are established. These cursor marks could be “dragged and dropped”to move them around the display, if desired. Components 60 alsopreferably include tools for assisting the operator in specifying thelocation of the trajectory guide alignment plane, and once the alignmentpoint has been identified, allow ready selection of a view of theimaging plane that allows alignment of the guide stem with the markedalignment point. Further, it is contemplated that the operator, oncehaving marked the target and the pivot point, can invoke the components60 to automatically display, in several simultaneous windows on thescanner display, images such as those illustrated in images of FIGS. 1B,2A and 2B, such that the stem 18 can be aligned with reference to thetrajectory guide alignment plane image, and simultaneously theconfirmatory images of FIGS. 2A and 2B (which can be altematinglyupdated), which further allowing the stem 18 to be removed and theinterventional device inserted to its desired location.

According to another aspect of the invention, there is provided anembodiment in which the positioning of the guide stem is accomplishedautomatically under control of the processing unit 54 and an electricalto mechanical linkage control 62, as illustrated in dotted-line form inFIG. 4. In this embodiment, software components 60 include computerinstructions to direct the electrical to mechanical linkage, through asuitable interface with the unit 54, to move the stem 18 until is alignswith the (x, y, z) coordinates of the alignment point, by comparing theposition of the marker 19 to the desired alignment point. The mechanicallinkage 64 may be of any design compatible with the imaging environment,such as disclosed in U.S. patent application Ser. No. 09/078913,identified more fully above.

Referring now to FIGS. 6 and 7, there are illustrated additionalalternative embodiments of the invention, wherein alignment of the guidestem is faciliated. According to one embodiment, processing unit 54outputs an audio signal 70 that indicates if the stem is getting closerto or further away from alignment, or at least an indication when thestem is properly aligned. This indication may be, for example, a beep ofvariable frequency, with one end of the frequency range indicating thefurthest away position, and the other end of the range indicatingalingment. Determination of alignment of the stem may be accomplished byimage analysis using unit 54. Alternatively, as illustrated in FIG. 7,the stem may include a micro-coil 72 which may receive a signal from asignal source 74, or may be passive and configured to resonate at knownfrequency. The processing unit 54 can in turn be configured to detect asignal generated by the coil 72, and determine the spatial coordinatesof the stem. FIG. 6 also illustrates a laser light control unit 80 whichreceives an output from unit 54 indicating the point in space the stem18 is to be aligned with. Unit 80 controls a pair of lasers 82 and 84,which may be directed by mechanical linkage or otherwise so as to directbeams 83 and 85 to interesect at the point (e.g. 34) in space in whichit is desired to align the stem 18. The stem 18 can then be readilyaligned to the intersection of the beams, without reference to the imagedisplay. Of course, alignment could then be verified with respect to theimage generated by the imaging apparatus. In another embodiment, otherlight sources could be used instead of laser light, such as infrared, orlight of other frequencies or other energy sources focused at a point inspace.

The invention further provides that the components 60 may beincorporated into the imaging system 50, or be distributed encoded in acarrier medium such as a magnetic media or digital data carried over anelectrical or optical medium.

Theoretical Basis of Calculations

Set forth below is one example technique for finding a line ortrajectory through two points in space graphically, as may be used inthe present invention. For this example, assume that a target and pivotpoints are denoted respectively as${\hat{r}}_{TP} = \frac{{\overset{\rightarrow}{r}}_{TP}}{{\overset{\rightarrow}{r}}_{TP}}$

For the purposes of the invention, the coordinates of these two pointsare measured using the imaging system. The desired trajectory is definedby the line connecting these two points (T and P), and mathematically isdefined as

{right arrow over (r)}−{right arrow over (r)} _(P) =k{circumflex over(r)} _(TP)

where r denotes a vector of any point along the line,${\overset{\rightarrow}{r}}_{T}$ ${\overset{\rightarrow}{r}}_{p}\quad$

r_(TP) denotes a vector from the target point T to the pivot point P,and k is a parameter that measures the distance along the line from thetarget point “T”.

In the frame of Cartesian coordinates, a plane can be generally definedas

αx+βy+γz=1

where αβγ are three parameters for the normal of the plane, whichsatisfy the following relationship.

 {square root over (α²+L +β²+L +γ²+L )}=1

Alternatively, a plane through a point (r₀) can be defined as

{right arrow over (r)}−{right arrow over (r)} ₀ =m ₁ {circumflex over(r)} ₁ +m ₂ {circumflex over (r)} ₂

where r denotes a vector of any point on the plane,${\hat{r}}_{i} = \frac{{\overset{\rightarrow}{r}}_{i}}{{\overset{\rightarrow}{r}}_{i}}$

(I=1,2) are two unitary vectors parallel to the plane, and m₁ and m₂ aretwo real numbers. The two unitary vectors can be chosen to be orthogonalto each other for convenience. If they are orthogonal, then the innerproduct of the two unitary vectors is zero as shown below

{circumflex over (r)} ₁ ·{circumflex over (r)} ₂=0

And the normal vector of the plane is given by the cross product asshown below

{circumflex over (n)}={circumflex over (r)} ₁ ×{circumflex over (r)} ₂

In general, a point on the plane can be expressed as follows

{right arrow over (r)}={right arrow over (r)} ₀ +m ₁ {circumflex over(r)} ₁ +m ₂ {circumflex over (r)} ₂

The equation above implies that each point on the plane correspondsuniquely to a pair of numbers (m₁ and m₂).

The intercept point between the line and the plane is given be thesolution of the following vector equation.

{right arrow over (r)} _(P) +k{circumflex over (r)} _(TP) ={right arrowover (r)} ₀ +m ₁ {circumflex over (r)} ₁ +m ₂ {circumflex over (r)} ₂

Solving this vector equation numerically, the corresponding m₁ and m₂ aswell as the parameter k for the point of interception can always beobtained. This absolute interception point can be translated to a pointon the scanned image which has a limited field of view (FOV) andresolution, and can be graphically displayed. When the image of thereference guide is adjusted to coincide with the in-plane target pointon the image, the reference guide is guaranteed to be in the samedirection defined by the two points (target and pivot ).

Thus, there has been described above method and apparatus forpositioning an interventional device in a body using a cross sectionalimaging system. Although the invention is described in one embodiment asa method of obtaining a biopsy in the brain of a human, it is in no waylimited to use in obtaining biopsies or for human use. Also, althoughthe invention was described above with respect to a MRI scanner, it isapplicable to any cross sectional imaging/scanning system, such as aCT-scanner, PET scanner or ultrasound scanner. The invention providesfor a quick and precise way to obtain the proper alignment of the guide,thereby improving the efficiency of use of the scanning equipment andspeeding the procedure and lessening the time of discomfort for apatient. Although the invention has been described in a preferred form,it shall be understood that many modifications and changes may be madethereto without departing from the scope of the invention as defined inthe claims appended hereto.

What is claimed is:
 1. A method of positioning a guide for aninterventional device in a body, wherein the guide pivots about a pivotpoint, comprising locating the spatial coordinates of a target and thepivot point, determining a third point outside of the body lying alongor proximate a line extending through the target and pivot point, andaligning an axis of the guide with the third point in an image taken inan alignment plane containing or substantially containing the thirdpoint, wherein the line does not lie in the alignment plane and whereinthe image obtained includes imaging information from a componentassociated with the guide such that the imaging information is used toalign the guide with the third point.
 2. A method according to claim 1wherein the alignment plane is orthogonal or substantially orthogonal tothe axis of the guide.
 3. A method according to claim 1 wherein theangle between the alignment plane and the line is at least great enoughto allow a MR visible component located proximate the axis of the guideto be viewed in an image taken in the alignment plane and so as topermit the MR visible component to be used to align the axis with thethird point in the image of the alignment plane.
 4. A method accordingto claim 3 further wherein the third point is marked on the image andthe axis of the guide is located by imaging at least a portion of analignment stem of the guide.
 5. A medical imaging system including aprocessing unit and computer software operative in the processing unitto permit an operator of the system to locate the spatial coordinates ofa target point and a pivot point of a guide for an interventional deviceto be located in a body, and automatically determine a third pointoutside of the body lying along or proximate a line extending throughthe target and pivot point, wherein the third point is located at adistance from the body allowing an image plane to be obtained in a spaceoutside the body and wherein the image plane contains or substantiallycontains the third point.
 6. A medical imaging system according to claim5 wherein the computer software is further operative on the processingunit to display an image taken on a guide alignment plane within orsubstantially within which the third point lies and which intersectswith an alignment axis of the guide, and further operative to display amark on the image which indicates the position of the third point toallow the alignment axis of the guide to be aligned with the third pointin the plane of the image.
 7. A medical imaging system according toclaim 6 further wherein the computer software is further operative onthe processing unit to assist an operator in obtaining an image by whichthe axis of the guide can be aligned with the third point.
 8. A medicalimaging system according to claim 7 wherein the computer software isfurther operative on the processing unit to allow the target and pivotpoint to be marked using a pointing device and to automaticallycalculate the line extending through the target point and the pivotpoint, and to further determine and display the image taken along theguide alignment plane.
 9. An article of manufacture comprising acomputer program encoded in a carrier, wherein the program is operativeon a processing unit of a medical imaging system to permit an operatorof the system to locate the spatial coordinates of a target point and apivot point of an interventional device to be located in a body, andautomatically determine a third point outside of the body lying along orproximate a line extending through the target and pivot point, whereinthe third point is located at a distance from the body allowing an imageplane to be obtained in a space outside the body and wherein the imageplane contains or substantially contains the third point.
 10. An articleof manufacture according to claim 9 further including a computer programoperative on the processing unit to assist an operator in obtaining animage by which the axis of the guide can be aligned with the third pointusing an imaging system.
 11. A method of positioning a guide for aninterventional device in a body, wherein the guide pivots about a pivotpoint, comprising locating the spatial coordinates of a target and thepivot point, determining a third point outside of the body lying alongor proximate a line extending through the target and pivot point, andaligning an axis of the guide with the third point by obtaining an imagein an alignment plane wherein the third point is in the plane and theaxis intersects the plane so as to permit the point of intersection ofthe axis and the third point to be aligned with one another in theimage, and wherein the image obtained includes imaging information froma component associated with the guide such that the imaging informationis used to align the guide with the third point.
 12. A method accordingto claim 11 wherein the interventional device is a biopsy needle or adrug delivery probe.
 13. A method according to claim 11 wherein the lineextending through the target and pivot point is substantially parallelto a long axis of a patient.
 14. A method according to claim 11 whereinthe line extending through the target and pivot point is substantiallyorthogonal to the long axis of the patient.
 15. A method according toclaim 11 wherein the images are determined using a computed tomographic(CT) scanner, an magnetic resonance imaging (MRI) device, or anultrasound imaging device.
 16. A method of positioning a guide for aninterventional device in a body, wherein the guide pivots about a pivotpoint, comprising locating the spatial coordinates of a target and thepivot point, determining at least a third and fourth points outside ofthe body lying along or proximate a line extending through the targetand pivot point, and aligning an axis of the guide initially with the atleast third point in an image taken in an alignment plane containing orsubstantially containing the at least third point, wherein the line doesnot lie in the alignment plane, and subsequently with the fourth pointoutside of the body lying along or proximate a line extending throughthe target and pivot point, and aligning an axis of the guide with thethird and fourth points and the target point, in such a manner that maycorrect for slight imprecision at the pivot point, such that an evenmore accurate trajectory can be achieved along the line connecting thetarget point, the pivot point and the at least third and fourth points,and wherein the image obtained includes imaging information from acomponent associated with the guide such that the imaging information isused to align the guide with the third point.